For the past 150 years, human society relied on fossil fuels initially for illumination purposes and later for automobile fuels and generation of electricity. However, usage of fossil fuels releases carbon dioxide to the atmosphere. Industrialization has raised the earth's atmospheric CO2 concentration from the pre-industrial level of 280 ppm to the current value of about 375 ppm. The ‘business as usual’ scenario, with no intervention, is expected to raise the CO2 concentration to about 750 ppm by the middle of this century. According to Hoffert et al., stabilization of atmospheric CO2 concentration at twice the pre-industrial concentration level will require 10 TW of additional carbon-emission-free power by 2050. Worldwide primary energy consumption in 2001 was 13.5 TW (1 TW=1012 watts) which is expected to increase to 27.6 TW by 2050. Best way to decrease the atmospheric CO2 concentration will be to reduce emission of CO2 to the atmosphere.
One energy sector where reduction of CO2 release is proving to be difficult is the transportation sector. Automobiles, trucks, planes etc. use gasoline and diesel as sources of energy. These liquid fuels are used due to their high energy density and ease of use. However, their combustion leads to release of massive quantities of CO2 and for this sector, CO2 capture is not viable. Therefore, alternate energy carriers like H2 and battery powered vehicles have been proposed. However, there are challenges associated with the use of H2 and battery powered vehicles.
Conventional H2 storage as compressed or liquefied hydrogen, hydrogen adsorption on large surface area, and hydrogen storage by metal hydrides have been proposed, but none of these strategies can currently meet the 8 wt % target for storing hydrogen required by the transportation sector. As a result, one major challenge in the use of H2 for the transportation sector is its storage, transportation and delivery.
Rechargeable Battery powered vehicles is another option proposed to reduce greenhouse gas emissions. A major challenge involved with batteries is that storage density of most commercial batteries is in the range of 75 to 150 Whr/kg, which is only sufficient for a short distance driving.
It is clear that liquid hydrocarbons are needed to fulfill the need for long distance driving and for aviation purposes because of the ease of handling and storage. However, most predictions suggest that conventional oil production will peak by 2025. It has been suggested to use biomass for the production of liquid fuels. This can also provide a solution to the problem of CO2 emission for transportation sector. This process is essentially CO2 neutral because CO2 released from vehicle exhaust will be captured from the atmosphere during biomass growth.
There are currently two major processes for conversion of biomass to liquid fuels. In the first process, fermentation of easily fermentable plant products such as sugar, sucrose, dextrose, xylose, etc. to alcohols is achieved. These easily fermentable plant products can be extracted from corn, sugar cane etc. Some of the alcohols thus produced that can be used as fuel are methanol, ethanol, butanols etc. Carbon dioxide is also liberated as a byproduct during the fermentation process. The major disadvantage of this pathway is that only a fraction of the total biomass is converted to the final desired liquid hydrocarbon fuels. As a result, such processes have extreme difficulty in meeting the large demands of the transportation sector. It has been estimated that only 12% of the total gasoline and 6% of the diesel demand can be fulfilled from all corn and soybean conversion to ethanol and biodiesel respectively in U.S. On the other hand, the second major process involves gasification and is capable of using whole biomass as feedstock for reaction instead of only a small fraction, as in the case of fermentation. This pathway involves gasification of biomass to obtain synthesis gas (syngas), a mixture of CO & H2 in any combination and conversion of this synthesis gas to liquid fuels using Fischer-Tropsch (FT) process. A quick estimate can be made for the required land area to support total current oil consumption of 13.84 million barrels per day by the U.S. transportation sector. For this purpose, the numbers for the amount of syngas production from biomass gasification provided in the recent National Research Council's report on H2 can be used. If one assumes the conversion of syngas to diesel to be 100% efficient then the land area requirement for the current biomass growth rate and gasification efficiency is estimated to be about 5,296,000 square km. This required land area is 58% of the total U.S. land area. To put the numbers in perspective, the currently used cropland area in the U.S. is 1,395,000 square km, which is roughly 20% of U.S. land area. Clearly, this is not a feasible solution.
A schematic of a typical biomass based gasifier process is shown in FIG. 1. Water content of biomass such as switch grass, corn, wood and other cellulosic mass is relatively high. Therefore, the biomass generally needs to be pre-dried prior to feeding it in a gasifier. Such a drying process can easily consume 10-20% of the total biomass as an energy source to supply heat for drying rest of the biomass. The dried biomass is then gasified to a mixture of CO, CO2, H2 and H2O (also known as synthesis gas) using oxygen and steam at temperature generally around 500-1500° C. and pressures of 1-100 bar. The ratio of H2 and CO in the synthesis gas is important and depends on the end use. H2/CO ratio should be (2n+1)/n for alkanes containing n carbon atoms per molecule and 2 for alkenes and alcohols. In a typical gasifier, oxygen is supplied to combust a portion of the feed stock. The resulting combustion energy not only supplies the energy losses from the system but the majority of it is stored in CO and H2 exiting the gasifier. The efficiency of a biomass gasifier is 50 to 70%, meaning that the thermal energy content of the gas exiting the gasifier is 50% to 70% of the thermal energy content of the biomass fed to the gasifier. Depending on the temperature and efficiency of the gasifier, CO2 concentration can vary from 6 to 29 mol % on a dry basis. The subsequent water-gas shift reaction adjusts the H2/CO ratio to about two, which is needed for the formation of straight chain hydrocarbons in a Fischer-Tropsch reactor. This leads to the formation of additional CO2 due to the Water Gas Shift reaction (WGS).CO+H2O═CO2+H2ΔH0° C.=−41.203 kJ/molΔH1000° C.=−32.196 kJ/mol
Using this reaction, CO concentration can be decreased and H2 can be increased to achieve the desired ratio. From the Fischer-Tropsch reactor, we get a distribution of products ranging from C1 to C100. Smaller, gaseous molecules can be burned to generate power and higher molecules are hydro-cracked to give diesel range molecules. However, the thermal efficiency of an actual biomass to synthetic hydrocarbon liquids using gasification and Fischer-Tropsch (FT) process is only about 35%. The carbon conversion efficiency of biomass to hydrocarbon liquid is also low. Depending on the overall efficiency of the process, nearly one third of carbon contained in the original biomass show up in the diesel molecules. Rest of the carbon in biomass is emitted back to the atmosphere as CO2. This disadvantage applies to biomass to methanol process as well as for any other biomass to liquid processes because gasification and WGS reaction is involved to obtain the required H2/CO ratio.
An advantage of using a gasifier operating at high temperatures and pressures for the production of liquid fuels instead of direct biomass liquefaction, biomass pyrolysis or any other such configurations are as follows: removal of pollutants from the gaseous stream at the exhaust of the gasifier is easy, gasification brings different forms of biomass to a common denomination (CO & H2) to obtain uniform fuel through liquid hydrocarbon conversion reactor. In contrast, the direct biomass liquefaction will give alcohols, aromatics, alkanes, alkenes or any combination of these compounds and separation processes to obtain them in desirable form are much more complicated. One of the major disadvantages of direct biomass liquefaction and biomass pyrolysis is that all the pollutants present in the biomass shows up in the product mixture and separation will be very difficult. Also, aromatics are formed in these processes. This presents challenge as the norms for sulfur and aromatics contents in liquid fuels are becoming more stringent. Furthermore, the composition of the reactor effluent will depend on the type of biomass used.
The unique advantage of FT diesel produced is that in addition to being high energy density liquid fuel which can be easily transported, it has low vapor pressure so there is no need to pressurize the storage tanks. This means that it can be transported at atmospheric pressure in normal tankers with very little loss during storage and transportation, unlike methanol. Synthetic diesel so produced is not soluble in water and it is biodegradable so that in case of any spillage during transportation, it can be degraded by micro-organisms. Other major advantage is that it is compatible with existing engines and fuel infrastructure. Also, diesel engines are 50% more efficient than gasoline engines.
As in the case of any chemical process, only a fraction of synthesis gas fed to the FT reactor reacts to form desired products and the unreacted synthesis gas is recycled back to the FT reactor after the separation of CO2. There has been an attempt in the past to recycle CO2 to the Fischer-Tropsch reactor and convert it to CO using the reverse Water Gas Shift reaction, but the conversion is limited by the thermodynamics of the reaction and the relatively fast nature of the CO hydrogenation reaction. H2 in the Fischer-Tropsch reactor is almost exclusively consumed by CO rather than CO2.